In the operation of modern aircraft, flight control surfaces such as slats and flaps are powered by primary movers known as power drive units. The power drive units generate torque which is transmitted via a variety of transmission means to move the flight control surfaces in desired directions depending on the navigational and other demands placed on the aircraft. Given the importance of the flight control surfaces to the safety of the aircraft and its passengers, it is critical that the flight control surfaces be controlled by a reliable actuation system.
One problem associated with prior art systems is that if the drive line or transmission controlling the flight control surface becomes mechanically jammed, or if flight conditions prevent movement of the control surface in the desired direction, the power drive unit will not stop generating torque. Rather, the power drive unit, which is normally hydraulically powered, will generate relatively high stall torque. This stall torque will be transmitted to the input shaft and gearing of the flight control surface actuator, and is often sufficiently high to detrimentally affect and potentially catastrophically damage the flight control surface or the flight control surface actuator.
Various attempts have therefore been made to devise a system to prevent such stall torque from being transmitted to the flight control surfaces in the event of such a mechanical jam or other interference. For example, U.S. Pat. No. 4,030,578, issued to Cacciola et al., discloses a brake disc pack which rotates with the actuator shaft and is compressed against a non-rotatable object as, for example, part of a housing when it is desired to stop rotation of the actuator shaft. The compression is against the bias of a spring and the excess torque is absorbed in the brake disc pack and grounded to the housing. One undesirable feature of such a system is that the predominantly controlling feature with respect to the amount of torque capable of being absorbed is the coefficient of friction exhibited by the brake disc pack. Therefore, in order to ensure the brake disc pack is sufficiently powerful to stop rotation of the actuator shaft, the actual brake discs are often oversized. This necessarily imposes substantial weight and space penalties on the aircraft. As modern aircraft are continually being redesigned and fine-tuned to reduce both the weight and volume of the onboard equipment, such brake disc packs are becoming increasing undesirable. It would therefore be desirable to eliminate such brake disc packs and optimize the weight and volume requirements even further, and therefore result in a more efficient aircraft.
A more recent attempt at such a reduction is disclosed in U.S. Pat. No. 5,299,666, issued to Lang et al., and assigned to the present assignee. As disclosed therein the brake disc pack is eliminated by employing a pair of toothed plates wherein one plate is splined to the actuation shaft for rotation therewith, and the other is fixedly attached to the actuator housing. When it is desired to stop rotation of the input shaft, the plate splined to the actuation shaft is moved into engagement with the plate attached to the housing. By employing toothed plates, the dependence on the coefficient of friction of the brake discs, as well as the weight and space penalties associated therewith, are avoided. In addition, the Cacciola et al. patented device employs a ball ramp coupling to transmit torque. With such ball ramp couplings, the balls need to be constructed of sufficient size so as to be capable of transmitting torque levels up to the level of stall torque. Such size requirements necessary impose negative weight and space penalties similar to those identified above which cannot be tolerated in modern, efficiency-conscious aircraft.